The overall objective of Project 3 is to quantify, and through improvements, maximize the precision with which target volumes within patients can be localized at the time of treatment. A fundamental goal of conformal radiation therapy is to confine regions of high dose as tightly as possible to target volumes. currently, immobile biologic target volumes need to be expanded to form planning target volumes to account for uncertainties in patient setup between treatments and for organ motion within patients. However, to escalate dose safely beyond current standards, it is necessary to minimize the volume of tissue irradiated. In this project it is hypothesized that through use of improved patient immobilization, real-time diagnostic and megavoltage flat-panel imagers, software to compute corrections for repositioning patients, and a computer controlled treatment machine, it will be possible to accurately and rapidly setup a patient before and/or during each treatment session. It is further hypothesized that, through use of a combination of radio-opaque markers visible on diagnostic and megavoltage studies, the influence of organ motion on target volume localization can be accounted for on a quantitative basis. Thus, Project 3 has two specific aims, which will be addressed through completion of three major subtasks associated with each aim. Aim 1: To develop and implement procedures that enable the reduction of "patient" setup inaccuracies to a level consistent with the precision with which patient position can be measured; with subtasks related to 1a) patient immobilization utilizing improved devices and techniques, 1b) patient localization by means of diagnostic x-ray sources located at the treatment unit in conjunction with innovative composite diagnostic- megavoltage, online flat-panel imagers, and 1c) computer-assisted patient repositioning using a redesigned computer-controlled table top system. Aim 2: To account for "target" motion due to patient organ motion in an otherwise immobilized patient; through subtasks which 2a) determine the stability of inter-treatment patient structure and organ localization, 2b) directly quantify target volume motion in the prostate, liver and lung using implanted radio-opaque markers, and 2c) employ advanced techniques to compensate for organ motion on a daily basis through adjustment of treated volumes or use of physiological gating techniques.